This invention relates in general to ion beam handling in mass spectrometers and more particularly to a means of focusing ions in time-of-flight mass spectrometers (TOFMS). The apparatus and method of mass analysis described herein is an enhancement of the techniques that are referred to in the literature relating to mass spectrometry.
The analysis of ions by mass spectrometers is important, as mass spectrometers are instruments that are used to determine the chemical structures of molecules. In these instruments, molecules become positively or negatively charged in an ionization source and the masses of the resultant ions are determined in vacuum by a mass analyzer that measures their mass/charge (m/z) ratio. Mass analyzers come in a variety of types, including magnetic field (B), combined (double-focusing) electrical (E) and magnetic field (B), quadrupole (Q), time-of-flight (TOF) mass analyzers, quadrupole ion storage trap, and, fourier transform ion cyclotron resonance (FT-ICR) mass analyzers, which are of particular importance with respect to the invention disclosed herein. Each mass spectrometric method has a unique set of attributes. Thus, trap and analyze type of mass spectrometers such as Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) arose out of the evolution of the larger field of mass spectrometry.
A number of ion sources can and are used in conjunction with trap-and-analyze mass spectrometers. Included among these is matrix assisted laser desorption/ionization (MALDI). The MALDI ion source has its origins in a work performed by M. Karas et al. in 1985 (M. Karas, D. Bachmann, F. Hillenkamp, Anal. Chem. 57, 2935(1985)). The observations of that work were developed into the MALDI method as described in later articles (M. Karas, F. Hillenkamp, Anal. Chem. 60,2301(1988)). When analyzing ions by MALDI-MS, sample is dissolved in a matrix of organic acid crystals. A laser is used to excite the organic acid matrix so that it sublimes into the vacuum of the mass spectrometer. It is important to note that the laser light used to excite the matrix is of a wavelength that the sample molecules do not absorb it. Thus, the sample molecules remain relatively cool throughout the desorption/ionization process. Also, the laser pulse used to excite the matrix is generally very short. Typically, the laser pulse duration is on the order of a few nanoseconds.
As the excited matrix sublimes, sample molecules are ejected into the vacuum as well. In the resulting plume, sample molecules can be ionized by, for example, proton transfer from the excited matrix molecules. In this way, MALDI can be used to produce ions from high molecular weight labile compounds such as proteins and other biological molecules (Hercules et al., Anal. Chem. 63, 450(1991)).
One of the difficulties with interfacing MALDI with mass spectrometry is related to the kinetic energy distribution that the ions have after desorption and ionization. The MALDI process results in the ejection of ions from the solid sample into the vacuum. The ions are ejected with a range of velocities and therefore kinetic energies. This distribution was measured in a work by Beavis and Chait (R. Beavis and B. Chait, Chem. Phys. Lett. 181(5), 479(1991)). In that work, Beavis found that all ions regardless of their mass-to-charge ratio have virtually the same velocity distribution. That is, a sample molecule of molecular weight 15,590 Da results in ions having nearly the same velocity distribution as molecules of molecular weight 1030 Da. The observed velocity distribution was centered at about 750 m/s and ranged from roughly 500 m/s to roughly 1000 m/s. This results in an initial ion kinetic energy distribution which is directly proportional to mass. For ions of about 1000 Da (roughly the mass of a peptide) the energy distribution would be on the order of a few eV. For ions of about 10,000 Da (small proteins), however, the energy distribution would be on the order of tens of eV.
MALDI sources have been used with varying degrees of success in conjunction with trap mass spectrometers. In the field of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS), for example, a Penning ion trap is used. The conventional Penning trap consists of six metal plates forming a cube in a magnetic field (M. B. Comisarow, Adv. Mass Spectrom. 8, 1698(1980); M. B. Comisarow, Int. J. Mass Spectrom. Ion Phys. 37, 251(1981)). Two of these plates (trapping plates) reside in planes perpendicular to the magnetic field whereas the other four (the excite/detect electrodes) are in planes parallel to the magnetic field. In conventional FTICR-MS, the trapping plates together with the magnetic field are used to trap ions. To accomplish this, a small electrical potential (e.g. 1 V) is applied to the trapping plates. The remaining plates are held at ground potential. The magnetic field confines ions in the plane perpendicular to the magnetic field line B, the x-y plane, and the electric field produced by the potential difference between the trap electrodes, and the excite/detect electrodes confines the ions along the magnetic field lines B, the z axis. It should be noted that ions from an external ion source, such as MALDI, enter the cell through an aperture in one of the trapping plates and initially are moving mainly along the z axis. Thus, the distribution in initial kinetic energies of the ions from a MALDI or other external ion source is directed along the instrument's z-axis.
In 1992, Wilkins et al. (J. A. Castoro, C. Koester, C. L. Wilkins, Rapid Commun. Mass Spectrom. 6, 239(1992)) used an FTICR mass spectrometer in the analysis of various compounds including myoglobin (MW.about.17,000 Da). To accomplish this they used a gated-trapping technique to decelerate MALDI ions so that they could be trapped in their Penning trap.
Solouki and Russell (T. Solouki, D. Russell, Proc. Natl. Acad. Sci. USA 89, 5701(1992)) have demonstrated effective trapping of high kinetic energy ions by using a collisional cooling process used in conjunction with a high trapping voltage. In these studies, MALDI ions were cooled through collisions with inert gas molecules in a small volume chamber before entering the FTMS cell. An electrostatic wire ion guide was also used to position ions along the exact center of the cell. In this way, ions up to 157,000 Da were trapped and detected (T. Solouki, K. J. Gilling, D. H. Russell, Anal. Chem. 66, 1583(1994)). However, mass resolution was low.
In 1995, Yao et al. (J. Yao, M. Dey, S. J. Pastor, C. L. Wilkins, Anal. Chem. 67, 3638(1995)) used a five-plate trapping method and successfully trapped and analyzed MALDI produced ions up to m/z.about.66,000 Da. Again, however, mass resolution was poor and deceleration potentials were required for the excite and detect electrodes.